For industrial catalysts, automobile exhaust gas purifying catalysts and many electric appliances, precious metals such as palladium, platinum and rhodium have been used. Since such precious metals are expensive and useful as resources, they have been recovered after use and reused, that is, recycled. In recent years, demands for the resources conservation are increasing, and importance of recycle of precious metals is more increasing.
To recover a precious metal, many methods such as a sedimentation separation method, an ion exchange method, an electrodeposition method, a solvent extraction method and an adsorption method have been developed, and among them, a solvent extraction method has been widely employed in view of the economical efficiency and operation property.
A solvent extraction method comprises an extracting step of subjecting an aqueous phase in which palladium ions are dissolved and an organic phase in which a palladium ion extracting agent is dissolved to liquid-liquid contact to extract the palladium ions into the organic phase side, and a back extraction step of back-extracting the palladium ions into the aqueous phase side by bringing the palladium ions which had been extracted to the organic phase side into contact with an aqueous phase in which a back-extracting agent is dissolved (for example, Patent Documents 1 and 2).
However, the solvent extraction method has problems in view of the safety and the environmental burden, since a large quantity of an organic solvent is used. Further, a disulfide compound such as dioctyl sulfide or dihexyl sulfide, commonly used as the extracting agent for the extracting method is likely to be oxidized, and accordingly its repeated use is problematic.
To solve such problems, a palladium separating agent has been proposed, which can be reused by improving the structure of the dialkyl sulfide by introducing an amide group to the vicinity of S thereby to prevent oxidation of the disulfide compound (for example, Patent Document 3).
However, even this palladium separating agent provides an insufficient selectivity of separation of palladium from a solution containing palladium ions at a high concentration, and it has been desired to develop a separating agent capable of separating palladium from a palladium ion solution of a low concentration to a high concentration in a short time with a high selectivity, and a method for separating palladium.